The objective of the application is to investigate the molecular mechanisms that underlie Tat and Rev regulation of HIV expression. The Sharp laboratory has recently identified a cellular factor, Tat-SF, that is required for Tat function. It was purified biochemically, using its intrinsic activity to monitor purifications. It has been microsequenced and the corresponding cDNA cloned. Tat-SF appears to assist in processivity of elongation by RNAP II in the presence of Tat, and it appears to interact with both Tat and TAR. The application proposes to further characterize Tat-SF-Tat interaction, and the role of these proteins in the processiveness of transcription in vitro and in vivo. The kinase that phosphorylates Tat-SF, and the components that further increase Tat-responsiveness in vitro will be further purified. The laboratory has also shown that the pol II elongation factor SIII increases polymerase processivity, and reduces Tat activation, suggesting that Tat may function through facilitation in the activity of SIII. The role of this factor in Tat activation will be further examined. Finally, the application also proposes to further study Rev regulation of RNA splicing, and the interaction of Rev with the nucleoporin protein Rab.